In recent years, a light-emitting element in which a light-emitting organic compound or a light-emitting inorganic compound is used as a light-emitting material has been actively developed. In particular, a light-emitting element called an electroluminescence (hereinafter referred to as EL) element, which has a structure in which a light-emitting layer containing a light-emitting material is provided between a pair of electrodes, has attracted attention as a next-generation flat panel display element because of its characteristics such as thinness, lightweight, high response time, and direct-current low-voltage driving. In addition, a display in which an EL element is used has a feature that it is excellent in contrast and image quality and has a wide angle of view. Moreover, because the EL element is a plane light source, application to a light source such as a backlight of a liquid crystal display and lighting is considered.
An EL element includes a pair of electrodes and a light-emitting layer which contains a light-emitting material and is provided between the pair of electrodes. When current flows in the light-emitting layer, the light-emitting material is excited, and then the EL element can emit light of a predetermined color. To make a large amount of current flow in the light-emitting layer is effective in increasing the luminance of the EL element. However, application of a large amount of current into the EL element increases power consumption. In addition, the application of a large amount of current in the light-emitting layer also accelerates deterioration of the EL element.
In view of the above, a light-emitting element in which a plurality of light-emitting layers are stacked has been proposed (e.g., Patent Document 1). Patent Document 1 discloses a light-emitting element in which a plurality of light-emitting units (hereinafter in this specification, the light-emitting unit is also referred to as an EL layer) are provided and the light-emitting units are separated by a charge-generation layer. More specifically, it discloses a light-emitting element in which a charge-generation layer formed of vanadium pentoxide is provided on a metal-doped layer functioning as an electron-injection layer of a first light-emitting unit, and further a second light-emitting unit is provided over the charge-generation layer. The light-emitting element disclosed in Patent Document 1 can emit light with a higher luminance than a light-emitting element having one light-emitting layer, when current having the same current density is applied to the elements.